In recent years, power storage devices, in particular lithium ion rechargeable batteries have become used extensively as a power source for electronic equipment such as portable phones, notebook computers, and etc., or as a power source for electric automobiles and electric power storage applications. In particular, laminate type batteries and square type batteries, which use laminated films such as an aluminum laminated film as an exterior member are equipped for many thin electronic devices such as tablet terminals, thin notebook computers, and the like. Since these batteries are thin, they might be easily deformed by small expansion and the like, and there is an issue that the deformation affects the electronic devices. Therefore, deformation of these batteries are to be suppressed.
The lithium ion rechargeable battery is basically composed of a positive electrode and a negative electrode containing a material which can occlude and release lithium, and a non-aqueous electrolytic solution composed of a lithium salt and a non-aqueous solvent, the non-aqueous solvent being a carbonate such as ethylene carbonate (EC) and propylene carbonate (PC).
Furthermore, as materials for the negative electrode of the lithium ion rechargeable battery, a lithium metal, metal compounds which can occlude and release lithium (a metal simple substance, oxides, alloys with lithium, and the like), and carbon materials. In particular, non-aqueous electrolyte secondary batteries using, for example, coke and graphite (artificial graphite or natural graphite) among the carbon materials are widely put to practical use. As the negative electrode comprising these carbon materials stores and releases lithium and electrons at an extremely low potential equivalent to lithium metal, there is a possibility that many solvents are subjected to reductive decomposition. Therefore, irrespective of the kind of the negative electrode, a part of the solvent in the electrolytic solution is reductively decomposed on the negative electrode and, due to deposition of the decomposed product, generation of gas, and swelling of the electrode, migration of lithium ions is interfered and there arises a possibility that battery characteristics deteriorate, in particular a possibility that battery characteristics such as cycle characteristics and the like deteriorate, particularly when the battery is used at high temperature and high voltage. Furthermore, lithium ion rechargeable batteries having a negative electrode which is made of lithium metal, alloys of lithium metals, simple substances of tin, silicon and the like, and oxides of the simple substances can provide a high initial capacitance, but on the other hand, sometimes causes the progress of particle size reduction during cycling so that it is known that the reductive decomposition of the non-aqueous solvent occurs at an accelerated rate in comparison with the negative electrode made of a carbon material, resulting in significant deterioration of battery performance such as battery capacity and cycle characteristics, and the battery deforms due to swelling of the electrode.
On the other hand, materials which can occlude and release lithium such as LiCoO2, LiMn2O4, LiNiO2 and LiFePO4, used as positive electrode materials, occlude and release lithium and electrons at a high voltage of 3.5 V or more based on lithium, and therefore there is a possibility that many solvents are subjected to oxidative decomposition, particularly when the battery is used at high temperature and high voltage. Therefore, irrespective of a kind of the positive electrode, there are concerns that a part of the solvent in the electrolytic solution is oxidatively decomposed on the positive electrode to cause a resistance increase due to deposition of the decomposed product and that gas is generated due to decomposition of the solvent to cause swelling of the battery.
Meanwhile, in electronic equipment loaded with a lithium ion rechargeable battery, power consumption keeps increasing and a battery capacity continues to become larger. Accordingly, heat evolution from the electronic equipment also becomes a factor to accelerate temperature rise of the battery. This makes an environment for the electrolytic solution easier to cause decomposition together with a shift of the set charging voltage of the battery to a higher value. Situations must be avoided that the battery becomes swollen due to gas generation caused by decomposition of the electrolytic solution and that the battery becomes unusable due to operation of a safety mechanism such as current cut-off and the like.
Further, multi-functionalization of electronic equipment also advances more and more, and there is a tendency for power consumption to increase. Therefore, there is a demand for making the capacity of the lithium ion rechargeable battery higher and, for that purpose, density of the electrode is increased and a wasteful space volume inside the battery is reduced to make a volume occupied by the non-aqueous electrolytic solution within the battery smaller. Therefore, the situation is such that battery performance, when the battery is used at high temperature and high voltage, is liable to be deteriorated by a small amount of decomposition of the non-aqueous electrolyte.
There have been proposed various non-aqueous electrolytic solutions. For example, WO 2005/091422 (PTL 1) proposes a non-aqueous electrolytic solution comprising formates such as phenyl formate, biphenyl formate, and the like, which is described to improve cycle characteristics, electric capacity, and storage characteristics of the battery. Furthermore, JP H7 (1995)-192762A (PTL 2) discloses a non-aqueous electrolytic solution comprising propyl fluoroformate, which is described to improve high load characteristics, low temperature characteristics, and cycle characteristics of the battery. Despite the presence of conventional non-aqueous electrolytic solutions including these, there is still a demand for a non-aqueous electrolytic solution which can realize a power storage device of high performance.